The uncertainty principle is a well-known axiom of quantum mechanics, which in large part gave rise to the understanding of quantum objects (e.g. molecules, atoms, photons, spins, etc.) via a wavefunction or waveform that probabilistically describes a plurality of possible states the quantum object may occupy. States may describe characteristics such as position, direction, energetic state, etc.
The probabilistic description gives rise to the concept of quantum superposition, whereby it is presently understood that, in an isolated environment, any given quantum object capable of occupying more than one state is capable of occupying plural states simultaneously.
For example, an atom may simultaneously occupy an excited state and a ground state, or be simultaneously located in two different physical positions. However, as exemplified by the famous “Schodinger's cat” thought experiment, the act of observation effectively collapses the probability function such that the observer will only ever observe one of the plurality of possible states. Which particular state the probability function effectively collapses into during a particular observation event depends upon the probability function describing the quantum object.
Superposition has been experimentally verified by applying a coherent driving oscillating field (e.g. electromagnetic radiation) to a two-state quantum object which was in ground state at initiation, i.e. at moment t=0. Subsequently, a measurement was taken after time t to determine state of the object. Over time, many experiments as described above are performed. When the result is averaged and plotted according to observed state versus time, an oscillating curve between the two discrete states is revealed. This curve has become known as a “Rabi oscillation”.
As delay t before making a measurement on a system increases, amplitude of oscillations is decreases, and eventually oscillations become non-observable; this effect is usually referred to as decoherence. Decoherence typically arises due to influence of forces other than those giving rise to the coherent quantum superposition. These “other forces” may be internal to the quantum object (e.g. nuclear spin within the quantum object) or external to the quantum object (e.g. interaction with another quantum object such as a photon or atom of a different type than the system of identical quantum objects, interactions with a surrounding environment, etc.). Decoherence occurs when a system interacts with its environment in a thermodynamically irreversible way. This prevents different elements in the quantum superposition of the total system's wavefunction from interfering with each other.
The practical consequence of decoherence is an apparent collapse of the probability function as the quantum nature of the system “leaks” into the environment. That is, components of the wavefunction acquire phase changes from interaction with their immediate surroundings. A total superposition of the global or universal wavefunction still exists (and remains coherent at the global level), but its ultimate fate remains an interpretational issue.
The superposition and decoherence phenomena have also been explored for systems of multiple, identical quantum objects. At initiation, all of the quantum objects simultaneously transition between the two discrete states, such that the corresponding Rabi oscillation curves are in phase. However, due to internal and/or external influence, individual quantum objects within the system of multiple quantum objects will spontaneously fall out of synchronization with the in-phase quantum objects. Over time if one performs measurements on a sample of objects to determine the state of each object, the result is that the Rabi Oscillations for the system as a whole appear as random noise rather than a coherent oscillating signal.
Decoherence remains a major challenge to very useful applications of quantum mechanics, for example in quantum computing. In this context, a quantum computer includes a plurality of quantum objects that are analogous to the “bits” of a classical transistor computer (also known as “qubits”). One significant advantage of quantum computing as compared to classical computing is the ability to leverage the quantum object's superposition to increase the computational power of the device. In brief, because qubits are probabilistic rather than discrete (as is the case for classical bits), a quantum bit can encode information in the form of both magnitude and direction of an n-dimensional vector (where n is the number of discrete states the qubit) whereas classical bits only encode information according to direction (magnetic bit orientation).
Accordingly, since decoherence causes the effective collapse of the probability function in a quantum computer, superpositioned qubits lose the computational advantage over classical bits upon experiencing decoherence. Thus, decoherence can be viewed as the loss of information from a system into the environment.
The precise reasons for this decoherence effects remain unknown, but generally point towards some causes that are responsible for athermal noise. Problematically, athermal noise can only be controlled to a certain extent by cooling. At a particular material-dependent temperature, further reduction in the temperature of the material does not produce a corresponding reduction in the athermal noise. It is believed that some atoms could have several closely-spaced equilibrium positions and can toggle between these positions. Another model assumes electron transitions between two close (in space and energy) states associated with defects or disorder. Yet another model assumes the existence of free spins associated with defects or dangling bonds on device surfaces, with spin re-orientation causing qubit decoherence or superconducting quantum interference device (SQUID) flux noise. See, for example, the discussion by Oliver and Welander (“Materials in superconducting quantum bits” Materials Research Society Bulletin 38:816-824 (2013)).
It is generally accepted that single-crystal substrates, as well as epitaxially-grown superconducting and dielectric films minimize the number of defects and hence the number of two-level systems and associated device noise. However, the exact microscopic origin and coupling mechanism are still a matter of ongoing debate. See, for example the discussion by Sendelbach, et. al. (“Complex Inductance, Excess Noise, and Surface Magnetism in dc SQUIDs,” Physical Review Letters; 103:117001-4 (2009)).
Conventional approaches to avoiding decoherence in quantum computing and other similar systems leveraging superposition of quantum objects to practical advantage have generally approached problem by attempting to isolate the system from external influence, such as thermal radiation and external quantum objects that tend to interfere with the quantum objects and cause decoherence as noted above. For example, many of the world's most sensitive devices operate at ultra-low temperatures where thermal noise is reduced and electronic phase coherence is increased. Still, even in the best superconducting qubit devices, the coherence time is currently insufficient to apply quantum error correction algorithms and thus to build an actual quantum computer.
In addition, many large computing centers and systems have recognized that current architectures are approaching a limit in the sense that advancing computational power beyond the next generation of systems will require unprecedented power supply. The power problem is of such scale that it is presently estimated that a nuclear power plant or equivalent energy source would be necessary to accomplish the desired increase in computational power of the next-generation supercomputer or large super-server.
One solution to this power problem is to use quantum computers. However, it is also believed that power consumption may be significantly reduced using classical computing architectures, but this result is dependent upon reducing the noise inherent to elements of the classical computer. Noise resulting from external and internal sources can be reduced by using physically isolated superconductors cooled to cryogenic temperatures (e.g. less than about 4 Kelvin).
In practice, the power required to keep superconducting computers cold and to perform a large volume of computations appears to be smaller than power required to run similar computations using room-temperature computer. However, due to athermal noise, cooling below a certain temperature does not result in lower noise, and makes it impossible to further decrease the amount of power required for large volume computations.
Accordingly, it would be beneficial to provide materials and systems with sufficiently low inherent noise and decoherence to accomplish quantum computing and/or classical superconductor elements such as microwave resonators, superconducting quantum interference devices (SQUIDs), etc. capable of operating as a superconducting computer while significantly reducing power consumption and/or cooling requirements exhibited by existing superconducting computer elements.